Conventional electrochemical devices such as batteries, are used widely in the world as a source of portable electrical power from direct current. Such battery devices provide the electrical power for everything from watches to automobiles and as a consequence great value is placed on the energy density or electrical storage capacity of these devices and their continued ability to provide an adequate supply of electrical current to the communicating electrical device. Currently the lithium ion battery is a preferred battery configuration due to its inherent ability to store and discharge a large volume of electrical power in relation to its volume and weight.
The conventional construction of lithium ion batteries features a positive and a negative electrode formed of active material on a metal substrate. The electrodes are then encased in a cell can or casing whereupon both electrodes release and absorb lithium ions during discharge and charge of the battery depending on the direction of current flow. During discharge of the battery, the active material in the negative electrode releases lithium ions which are absorbed by the positive electrode and this process is reversed during charging of the battery whereby the positive electrode releases lithium ions to be reabsorbed by the negative electrode.
Continued function of batteries requires that a separator be placed between the positive electrode preventing it from direct contact with the negative electrode. Conventional lithium ion batteries achieve this separation using a separator made from a porous membrane made from a suitable dielectric material to maintain the desired degree of separation of both electrodes and allow passage of the electrolyte therebetween during the ion exchange when used.
An existing problem with the manufacture of conventional lithium ion electrodes arises from metal shards that are produced during the slitting process involved in forming the electrodes to the proper dimensions for encasement. These shards pose a constant danger of damage to the separator and potential shorts between the two electrodes resulting therefrom.
Both electrodes also communicate with a current collector which communicates the electrical current generated during discharge of the electrodes of the battery and permits the electrodes to communicate with an external power source which can be used to recharge the battery. Such current collectors further communicate battery developed electrical current to the external device which uses the battery as source of electrical power. Conventional current collectors are frequently manufactured from stainless steel, iron and nickel alloys, aluminum, copper, and similar materials that provide good electrical communication between the load drawing power external to the battery casing and the electrode housed internally.
As a medium to move the ions between the electrodes an electrolyte is used. A conventional electrolyte in a lithium ion battery contains lithium ions which move between the electrodes during charging and discharging. Such electrolyte provides the means of transport of the lithium ions through the porous separator which separates the two electrodes formed of active material. Typical electrolytes will not function above 150° C.
The constant flow of ions in the proximity of the current collector during discharge and charging of the battery has a high potential to cause corrosion of the current collector itself and the contact points with the electrode thereby degrading the communication of electrical current from the adjacent active material of the electrode. Such corrosion is caused by a number of factors in a lithium ion battery including but not limited to low corrosion resistance of the current collector, high temperatures caused by charging and discharging, the inherent nature of lithium being highly reactive, and other interrelating factors. Conventionally, aluminum and copper are especially susceptible to corrosion from the salts in the electrolytes in a lithium ion battery. Further, there is also a susceptibility to corrosion of current collectors of conventional batteries which can be attributed to inhomogeneous current flow through the current collector itself once it has become corroded and thereby come into uneven contact with the adjacent electrode.
An additional problem with conventionally manufactured lithium ion batteries is the requirement that once manufactured, they must be charged. Typically to prevent corrosion of the copper current collector, batteries must be charged to at least 10% of capacity so that they may be stored and shipped to customers. This is an expensive and time-consuming step in manufacture.
In dealing with lithium ion batteries there is the further vexing problem of combustion of the highly reactive materials forming the electrodes. The material forming the electrodes has a propensity to catch fire if the battery is overcharged or charged at the wrong voltage or overheated by an internal or external short.
As can be discerned, it is imperative that corrosion between the termination collectors adjacent to communicating active material forming the electrodes be kept to a minimum to avoid loss of efficiency of the battery. Such corrosion also risks the total failure of the battery should contact be lost between the termination collector such as the current collector or the battery case, and its communicating electrode. It is further highly desirable to eliminate the potential of shards and debris from the manufacturing process causing short circuits between the electrodes by circumventing the separator. Finally the reduction of the potential of fire during overcharge or short circuits internally or externally is highly desirable.
One solution attempted to prevent such fire hazards from short circuits has been the use of a non flammable electrolyte in the battery. Such a teaching of the use of a flame retardant electrolyte is found in U.S. Pat. No. 6,040,091 (Hiroaki) and U.S. Pat. No. 5,714,277 (Kawakami) as well as other patents. However placement of a flame retardant in the electrolyte as taught by these and other patents inherently complicates the chemistry of the battery since it renders the cell sensitive to voltage and the cell can decompose, as well as limiting the stability of the electrolyte itself.
U.S. Pat. No. 5,547,782 (Dasgupta) attempts to solve the problem of corrosion of the current collector by using an electrically conductive ceramic layer or electrically conductive polymer in a stacked relationship between the electrodes and the metal cover and case forming the battery container. However, Dasgupta also teaches the requirement that the polymer layer must be continuous and non-porous to prevent the severe corrosion caused by contact of the electrodes with the metal surfaces protected. This makes Dasgupta more expensive and not easily manufactured and fails to address the problem of shards and debris contaminating the electrodes and the need to pre-charge the battery before shipment to avoid corrosion.
U.S. Pat. No. 5,187,033 (Koshiba) teaches a lithium secondary battery using a porous film separator and a gold plated stainless steel current collector and uniquely formed electrodes. However Koshiba fails to address the issue of shards piercing the separator and the potential for corrosion with the metal current collector and the metal powder used therein, the fire hazard, and would still generally require a charging of the battery before storage and shipment.
There is a pressing need as such, for a method and components for use in battery construction which will provide for maximum communication of current from the electrode to the adjacent termination component and concurrently provide maximum resistence to the corrosion threatening that communication between the electrode and the termination component. Such a method and components used therein to form the resulting battery, should provide maximum resistance to such corrosion by maximizing the contact of the termination component with the adjacent electrode and should be easy to include in battery construction and be inexpensive. Further, a battery so formed, should minimize or eliminate the hazards of metallic shards causing short circuits and minimize the potential from fire in the battery. Additionally, such a battery should maintain thermal stability at high temperatures and should not be required to be pre-charged before shipment.